This disclosure relates generally to the field of digital photography. More particularly, this disclosure relates to the generation and use of a highlight exposure metric whose value is sensitive to the capture of an image's highlight information. The disclosed metric may be used, for example, to determine when high-dynamic-range (HDR) operations may be appropriate, the appropriate exposure value that will capture a scene without perceptual highlight loss, or to adjust the exposure ratio value between long- and short-exposure images during HDR operations. As used in this disclosure, the terms camera, digital camera, image capture system or device, digital image capture system or device, and the like are meant to refer to any device, component, element or system that has the ability to capture digital images (still and video).
Today, many personal electronic devices come equipped with digital cameras. Illustrative personal electronic devices include, but are not limited to, mobile phones, personal data assistants, portable music players, laptop computer systems, desktop computer systems, tablet computer systems and wearable electronic devices such as watches and glasses. Image sensors used in these types of devices often have relatively small dynamic ranges. That is, their ability to capture a range of light from total darkness to full sunlight in a single image is limited. Consumer-grade sensors often provide only 8-10 bits resolution. Such sensors can distinguish between 256-1024 different brightness levels (assuming no noise); generally an insufficient number to adequately represent the range of light to which they are exposed. One result of a scene exceeding a sensor's dynamic range is that pixels are clipped. As the number of clipped pixels increases, there comes a point where the image becomes unpleasant to look at.
High-dynamic-range imaging (HDRI or HDR) is a set of techniques used in digital capture systems to reproduce a greater dynamic range than is possible using standard single image capture techniques. HDR techniques often use exposure bracketing to capture multiple images at multiple different exposure values which are then merged into a single HDR image. While providing images with an extended dynamic range, these operations do not come for free. The decision as to whether to employ HDR in a given situation can be complex and is often a compromise between several factors including the computational complexity of implementation, noise, highlight recovery and the introduction of ghosting artifacts. In HDR operations, it is important to cover just the dynamic range needed to capture the scene—and no more (additional dynamic range capability merely tends to decrease the captured image's quality).